In pressing operations of the brake pedal of a vehicle, a press-type switch has been popular in recent years. Through the switch, the brake light is controlled so as to turn on when the brake pedal is pressed and turn off when the pressing force is removed.
First will be described such a conventional switch with reference to FIG. 2. FIG. 2 is a section view of a conventional switch. Case 21 is an insulating resin-made box having an opening on its top. Operating part 22 is also made of insulating resin. On the inner wall of case 21, a plurality of fixed contacts 23 and fixed tab 24 are disposed. Operating part 22 is accommodated in case 21 so as to be movable in the vertical direction.
Movable contact 25 is made of conductive metal. A coil-shaped pressure spring 26 is disposed, in a little contracted state, between movable contact 25 and the bottom of case 21. Urged upwardly by pressure spring 26, movable contact 25 contacts with fixed contacts 23. Fixed contacts 23 are thus electrically connected with each other via movable contact 25.
Conductive metal-made contact strip 27 is disposed in a little contract state in a way that one end is fixed to a side surface of operating part 22, and the other end contacts with a part of the right-side inner wall of case 21 placed upper than fixed tab 24.
Return spring 28 has a coiled shape. Cover 29 covers over the top opening of case 21. Return spring 28, which is disposed in a little contract state between the bottom of operating part 22 and the bottom of case 21, urges operating part 22 upwardly.
Cover 29 has hollow cylinder 29A that protrudes upwardly. Operation rod 22A of operating part 22 is disposed movable through the hollow of the cylinder in the vertical direction, with the top end of operation rod 22A exposed from the cylinder.
As operating part 22 moves in the vertical direction, contact strip 27 slides on fixed tab 24. To encourage a smooth sliding, lubricant 20 is applied to the contact surface between them. Lubricant 20 contains olefin-, or ester-base oil and lithium-, calcium-, or aluminum-based metallic soap.
The switch structured above is placed, in general, upstream from the brake pedal of a vehicle, with operation rod 22A of operating part 22 pressed by an arm (not shown). Terminal section 23A of fixed contacts 23, which protrudes from the bottom of case 21, is connected to a brake light via a connector, while terminal section 24A of fixed tab 24 is connected to electronic circuitry of a vehicle.
While no pressing force is applying to the brake pedal, operation rod 22A of operating part 22 is pressed downwardly. This pressing force contracts pressure spring 26 and return spring 28, lowering movable contact 25 away from fixed contacts 23. That is, movable contact 25 and fixed contacts 23 have no electrical connection; the brake light maintains turn-off state.
In the state, contact strip 27 fixed on the side of operating part 22 goes downward and slides on fixed tab 24 to establish electrical connections therebetween. That is, the electronic circuitry can provide the vehicle with “cruise control” by which the vehicle runs at a constant speed without being stepped the gas pedal.
When the brake pedal is stepped down, the arm goes away from operation rod 22A and therefore rod 22A gets free from the pressing force. Elastic return force of return spring 28 pushes up operating part 22. At the same time, the return force of pressure spring 26 moves movable contact 25 upward to make contact with fixed contacts 23. A plurality of fixed contacts 23 thus establishes electrical connections, so that the brake light turns on.
In the state, contact strip 27 slides on fixed tab 24 and away from it; no more electrical connections and accordingly, the electronic circuitry stops the cruise control.
Fixed contacts 23 and movable contact 25, which are responsible for turning on/off of the brake light, carry a relatively large current of several amperes on 12 volts d. c., whereas fixed tab 24 and contact strip 27, which are responsible for connecting/disconnecting the electronic circuitry, carry a small current of several milliamperes. As described above, lubricant 20 is applied between fixed tab 24 and contact strip 27. On the other hand, no lubricant is provided between fixed contacts 23 and movable contact 25 where an arc occurs in switch-on/off operations due to large current flow.
If lubricant 20 provided between fixed tab 24 and contact strip 27 accidentally attaches to fixed contacts 23 and movable contact 25, reliable switch-on/off operations of the electronic circuitry can be hindered. Considering this, lubricant 20 is, as described earlier, usually formed of olefin- or ester-based oil, not formed of silicon-based material that is easy to generate silica having insulating property by arc.
As a prior-art, for example, the structure disclosed in Japanese Patent Unexamined Publication No. 2001-84867 relates to the present invention.
In the conventional structure, however, lubricant 20 provided between fixed tab 24 and contact strip 27 contains lithium-, calcium-, or aluminum-based metallic soap. In the repeatedly performed sliding operations of contact strip 27 and fixed tab 24 in response to the pressing operation on operating part 22, the metallic compounding agent in the lubricant has gradually worn contact strip 27 and fixed tab 24, and eventually fails to provide a reliable connection therebetween.